This invention relates to dielectric block filters for radio-frequency signals, and in particular, to dielectric block resonators suitable for use in filtering signals generated in wireless communication applications.
Ceramic block filters offer several advantages over lumped component filters. The blocks are relatively easy to manufacture, rugged, and relatively compact. In the basic ceramic block filter design, the resonators are formed by cylindrical passages, called holes, extending through the block from the long narrow side to the opposite long narrow side. The block is substantially plated with a conductive material (i.e. metallized) on all but one of its six (outer) sides and on the inside walls formed by the resonator holes.
One of the two opposing sides containing holes is not fully metallized, but instead bears a metallization pattern designed to couple input and output signals through the series of resonators. This patterned side is conventionally labeled the top of the block. In some designs, the pattern may extend to sides of the block, where input/output electrodes are formed.
The reactive coupling between adjacent resonators is dictated, at least to some extent, by the physical dimensions of each resonator, by the orientation of each resonator with respect to the other resonators, and by aspects of the top surface metallization pattern. Interactions are complex and difficult to predict.
These filters may also be equipped with an external metallic shield attached to and positioned across the open-circuited end of the block in order to cancel parasitic coupling between non-adjacent resonators and to achieve acceptable stopbands.
Although such RF signal filters have received wide-spread commercial acceptance since the 1970s, efforts at improvement on this basic design continued.
In the interest of allowing wireless communication providers to provide additional service, governments worldwide have allocated new higher RF frequencies for commercial use. To better exploit these newly allocated frequencies, standard setting organizations have adopted bandwidth specifications with compressed transmit and receive bands as well as individual channels. These trends are pushing the limits of filter technology to provide sufficient frequency selectivity and band isolation.
Coupled with the higher frequencies and crowded channels are the consumer market trends towards ever smaller wireless communication devices (.e.g. handsets) and longer battery life. Combined, these trends place difficult constraints on the design of wireless components such as filters. Filter designers may not simply add more space-taking resonators or allow greater insertion loss in order to provide improved signal rejection.
Therefore, the need continues for improved RF filters which can offer selectivity and other performance improvements, without increases in size or cost of manufacturing. This invention overcomes the size-to-selectivity compromise by providing a ceramic block RF filter having adaptable selectivity with a robust, relatively low cost control mechanism and relatively low insertion loss.
An RF signal filter suitable for use in a mobile communication device is provided. The filter includes an elongate block of dielectric material having a transmit electrode and an antenna electrode in the form of selective metallization on the outside surfaces of the block. The dielectric block defines an array of spaced resonators extending between the transmit electrode and the antenna electrode, and a trap resonator adjacent to the transmit electrode but opposite the array of spaced resonators (i.e. at the end of the block nearest the transmit electrode).
The filter also includes a local ground conductive layer on the elongate block, an isolated electrode positioned between but spaced apart from the trap resonator and the local ground conductive layer, and a switch operably connected between the isolated electrode and the local ground conductive layer.
The switch allows shifting of the resonant frequency of the trap resonator by adding a predetermined capacitance of the isolated electrode. In a preferred embodiment, the switch is a PIN diode with a first terminal connected to the isolated electrode and a second terminal connected to the local ground conductive layer. The isolated electrode is adapted for connection to an external input source of DC biasing for switching the PIN diode from a conducting state to a non-conducting state. Alternatively, the isolated electrode is connected through a resistor to the transmit electrode such that a DC bias to the transmit electrode can be used to switch the PIN diode.
Another embodiment of this invention is a dielectric block filter adapted for use as an antenna duplexer. The duplexer comprises an elongate ceramic block which includes an antenna electrode on the elongate ceramic block and a transmitter branch extending between the antenna electrode and a first end of the block, a receiver branch extending between the antenna electrode and a second end of the block. Each branch has a plurality of through-hole resonators.
The ceramic block has a pattern of metallization on the outside surface of the ceramic block. The metallization pattern defines a plurality of connection electrodes including a transmit electrode, a receive electrode, an antenna, and a control electrode. The transmit electrode is spaced apart from the antenna electrode along a length of the block and positioned in the transmitter branch. The receiver electrode is spaced apart from the antenna electrode along the length of the block and positioned in the receiver branch.
At least one of the plurality of resonators of the transmitter branch is a trap resonator which is positioned between the first end of the block and the transmit electrode. An isolated electrode is positioned between but spaced apart from the trap resonator and the local ground conductive layer for creating a capacitive coupling between the trap resonator and the isolated electrode. The filter also includes a PIN diode mounted to the block that has a first terminal connected to the isolated electrode and a second terminal connected to the local ground conductive layer.
The filtering characteristics of dielectric block filters of this invention can be moved from one of two settings by applying and removing a biasing voltage to the control input. Specifically, the effect of the trap resonator on the transfer function of the filter can be shifted between two discrete positions by applying or removing a DC voltage to the control input.
There are other advantages and features of this invention which will be more readily apparent from the following detailed description of the preferred embodiment of the invention, the drawings, and the appended claims.